1. Field of the Invention
The present invention relates to polyphenylene ethers that have reactive end groups.
2. Description of the Background
Polyphenylene ethers (PPE) constitute a known class of thermoplastic structural materials that are distinguished by high-temperature dimensional stability and resistance to hot water, acid, and alkalies. Their preparation by oxidative coupling is described in detail in the patent literature (cf. U.S. Pat. Nos. 3,306,874; 3,306,875; European Patents 0 098 929, 0 099 965, 0 122 394, 0 137 139, and German Patent Application Disclosure 34 42 141). However, the extensive chemically inert nature of this polymer is a drawback for many applications such as, for example, blends with polyamides, and in such cases it is desirable for the PPE to contain a certain minimum quantity of reactive groups.
There are basically four possible ways for introducing functional groups in polyphenylene ethers:
I. The phenolic end groups of the unfunctionalized polyphenylene ether are reacted with suitable reagents such as anhydrides or acid chlorides (cf. German Patent 25 05 329 and WO 86/02 086). PA1 II. The alkyl groups of the phenol in the 2- and 6-positions in the polyphenylene ether are chemically modified so that they become functional groups themselves. PA1 III. A 2,6-dialkylated phenol is copolymerized with an appropriate phenol that has a functional group in the 4-position. It is well known that the molecular weight of the polyphenylene ether can be controlled in this way (cf. German Disclosure 17 45 201). PA1 IV. A 2,6-dialkylated phenol is copolymerized with an appropriate functionalized phenol that has no functional group in the 4-position. PA1 (a) for the synthesis of the oxazoline and hydroxyethylamide compounds above where n=0, the sodium or potassium salt of 2,6-dimethylphenol is carboxylated, and then reacted with ethanolamine either directly or through the methyl ester. By treatment of the hydroxyethylamide compound with thionyl chloride and then with aqueous sodium bicarbonate solution, conversion to the oxazoline derivative can be achieved (cf. V. Percec et al., J. Polym. Sci., Polym. Lett. Ed. 22, 523-532 (1984)); PA1 (b) for the synthesis of the oxazoline compounds where n=1 and R is methylene or --C.sub.2 H.sub.4 --, 4-acetyl- or 4-propionyl-2,6-dimethylphenol is reacted with sulfur and an amine by the Willgeroth-Kindler method, and then the carboxylic acid obtained is derivatized (cf. E. Schwenk, D. Papa, J. Org. Chem. 11, 798 (1946)); PA1 (c) for the synthesis of the oxazoline and hydroxyethylamide where n=1 and R is substituted --C.sub.2 H.sub.4 --, 2,6-dimethyl-phenol is reacted with acrylic acid or a substituted acrylic acid derivative under Bronsted or Lewis acid catalysis (electrophilic substitution in the para-position), and then the carboxylic acid is derivatized (cf. L. J. Smith et al., J. Am. Chem. Soc. 65, 282, 287 (1943)); and PA1 (d) for the synthesis of the oxazoline and hydroxyethylamide compounds similar to (c), other unsaturated but nonconjugated carboxylic acids or their derivatives can also be used if they are able to add electrophilically to phenols such as the Diels-Alder adduct of isoprene and acrylic acid (see European Disclosure 0 106 799).
Process embodiment I is limited in application to compounds that are able to react with the slightly reactive phenolic end groups present in great dilution.
In the case of process embodiment II, the alkyl groups in the 2- and 6-positions enter into a chemical reaction with even greater difficulty. In practice, one is limited to the use of halogens and strong oxidizing agents that are able to extract a hydrogen atom from the alpha-carbon atom. For example, Japanese Disclosure 86/066 452 describes a process in which a polyphenylene ether is reacted in the melt with maleic anhydride and peroxide. The method presents problems, since the anhydride is volatile and very toxic under these conditions.
Another technique of introducing functional groups in polyphenylene ether is described by Percec et al in which oxazoline groups are distributed statistically along the polyphenylene ether chain. The side chains of PPE are first brominated and a phase transfer catalyzed etherification with the sodium salt of 2-(p-hydroxyphenyl)oxazoline is then carried out. The process requires a double change of solvent and does not provide bromine-free products (cf. Polymer Bulletin 12, 261 to 268 (1984)).
The oxidative coupling of phenols is well known to provide high-grade polymeric products only when the redox potentials of the phenol and of the catalyst are carefully matched to each other. For this reason and for practical reasons, 2,6-dimethylphenol is used almost exclusively in practice. Functional groups drastically change the oxidation potential of the phenol monomer. In fact, it is uncertain whether such functional phenols are incorporated during the polycondensation reaction at all. Copolymerization reactions such as described above in techniques III and IV must therefore be considered to be problematical.
Ortho- and meta-linked polyphenylene ethers that can be obtained by coupling p-substituted phenols of the formula: ##STR2## under pressure are described in German Patent specification 34 14 882. Substituent X in this formula is a halogen or almost any organic group that may also carry functional groups. For example, X is a substituted alkyl group of 1 to 20 carbon atoms or
an aromatic oxazolyl group. The groups Q, Q', and Q" stand for hydrogen or a group with a maximum of 3 carbon atoms. However, because of their heterogeneous structure, characterized by severe branching and high nonuniformity and the different type of linking, these condensation products have only very remote similarity to the p-linked, "classical" polyphenylene ethers. A need therefore continues to exist for a polyphenylene ether which has a sufficiently high molecular weight whose functional end groups provide good compatibility with polyamides.